This invention relates to a release liner and a pressure-sensitive adhesive sheet comprising the release liner as a constitutional member. In particular, the present invention pertains to a release liner suitably employed for the assembly of an electronic device such as magnetic recording device (HDD) easily damaged by contamination of silicone.
Pressure-sensitive adhesive sheets such as pressure-sensitive adhesive double bond tape used for the bonding of various goods have a release liner stacked on the surface of its adhesive layer in order to protect the adhesive layer until actual use. As such a release liner, ordinarily employed is that obtained by applying, as a releasing agent, a silicone-base releasing agent to a base material such as paper or plastic film.
In recent days, pressure-sensitive adhesive sheets have come to be used even for the fabrication of precision electronic parts (devices) such as magnetic recording device (HDD). When used for such a purpose, removal of a silicone-base material from the constitutional members of the pressure-sensitive adhesive sheet is required for preventing contact point failure or head crash failure of a magnetic disc.
Among the constitutional members of a pressure-sensitive adhesive sheet, a silicone-base material being used most frequently is a silicone-base releasing agent as a releasing agent for a release liner etc. Accordingly, the most effective means to satisfy the above-described requirement is not to use a silicone-base releasing agent.
As means free from the use of a silicone-base releasing agent, use of a fluorine-base releasing agent or long-chain alkyl base releasing agent for a release liner or use of, as a release liner, a plastic film made of a polyolefin having a low adhesive force such as polyethylene or polypropylene can be considered. The use of a fluorine-base releasing agent is however accompanied with the problem that since it is expensive, cost of the release liner obtained using it becomes high. The use of a long-chain alkyl type releasing agent is also accompanied with the problem that a higher force is required for peeling off the release liner having it compared with a release liner having the silicone-base releasing agent.
It has already been found that in the case where a polyolefin-base plastic film is used as a release liner, some plastic films having a specific composition can be used as a release liner which requires only a low peeling force (excellent releasing function), but they tend to involve a problem in heat resistance. For example, when heat is applied to a release liner in the production step of a pressure-sensitive adhesive sheet, such a release liner tends to curl and its original shape cannot be maintained easily, which makes it difficult to maintain the appearance quality of the pressure-sensitive adhesive sheet. When heat is applied to a release liner in the assembly step of parts or the like, deformation of the pressure-sensitive adhesive sheet occurs on the line of the automatic assembly step, leading to a step failure. In addition, when a polyolefin-base plastic film used as a release liner or a pressure-sensitive adhesive sheet having it as a constitutional member is stored in the rolled form, adhesion (blocking) occurs between the release liners, resulting in difficulty in unwinding.
An object of the present invention is therefore to provide a release liner which exhibits good release function even without using a silicone-base material such as a silicone-base releasing agent and has heat resistance sufficient for preventing deformation such as curl even when heat is applied; and a pressure-sensitive adhesive sheet comprising the release liner as a constitutional member.
Another object of the present invention is to provide a release liner which has the above-described properties and in addition, does not cause blocking easily even when stored in the rolled form; and a pressure-sensitive adhesive sheet comprising the release liner as a constitutional member.
With a view to attaining the above-described objects, the present inventors have carried out an extensive investigation. As a result, it has been found that a release liner having good releasing properties and high heat resistance can be obtained even without using a silicone-base material, by forming its release layer to have a laminated structure of three layers and constituting each layer from a film layer having specific physical properties, leading to the completion of the present invention.
In one aspect of the present invention, there is thus provided a release liner which comprises a plastic film having a laminated structure of at least three layers, wherein the two surface layers of the release liner are (A) a release functioning layer composed of a polyethylene film having a density of 0.88 to 0.92 g/cm3 and a melt index not greater than 10 g/10 min. and (B) a surface film layer having a thermal linear expansion coefficient similar to that of the release functioning layer; and the intermediate layer is (C) a reinforcement layer having a melting point not less than 120xc2x0 C.
In another aspect of the present invention, there is also provided the above-described release liner, wherein the surface film layer (B) is formed from a material different from that of the release functioning layer (A). In a further aspect of the present invention, there is also provided a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer formed on the surface of, the release functioning layer (A) of the above-described release liner. The modulus of elasticity of the pressure-sensitive adhesive sheet at 23xc2x0 C. is, for example, about 1xc3x97104 to 1xc3x97106 dyn/cm2. This pressure-sensitive adhesive sheet can be used, for example, as a pressure-sensitive adhesive sheet for a hard disc device of a computer.
In the present invention, since a release liner has a laminated structure of at least three layers and each of these three layers is composed of a film layer having specific physical properties, it exhibits a good release function even without using a silicone-base material and at the same time, has excellent in processing properties and heat resistance so that deformation such as curl does not occur easily even when heat is applied to it. It can therefore be used suitably for the bonding of electronic parts easily damaged by contamination of silicone. In addition, by forming the release functioning layer and surface film layer, which constitute two surface layers of the release liner, from different materials, blocking of the release liner in the rolled form can be prevented effectively.